lm368 driving motors

Discussion in 'General Electronics Chat' started by chaos51, Jun 20, 2012.

  1. chaos51

    Thread Starter Member

    Jun 18, 2011
    39
    1
    Hi,

    This is a little about beam robotics, and why it works. There is namely a simple to buildbeam robot called herby.

    It's brain is build on a lm368 audio amplifier.

    The circuit is simple, see for example: Here

    The question now is, looking at the datasheet of the lm368, I cannot really find out where in the datasheet it says it can crank out somuch current it can run a motor.

    I build the circuit myself, on different motors,and it seems to work very fine. The LM is not overheathing or anything, and there is enough oomph to push a small robot.... So even if the lm368 is not intended to do this, it obviously can.

    Looking at the datasheet
    This makes not much sense to me.... But then again I don't see the datasheet saying out loudly "Max output current"... Maybe it is the quiescent current I should be looking at? I was expecting something to say 1A, or 5W... But I see much smaller figures.

    I know it's a simple question, but I am not sure, and would like to hear what you guys think.

    Regards
    DaC
     
  2. #12

    Expert

    Nov 30, 2010
    16,261
    6,770
    I think the first thing to look at is whether you are using a voltage reference chip or an audio amplifier.
     
  3. Audioguru

    New Member

    Dec 20, 2007
    9,411
    896
    The circuit uses an LM386 audio amplifier IC, not an LM368. (Dyslexic?)
    The datasheet shows its output is 6V peak to peak into an 8 ohm load with a 9V supply so the peak current is 3V/8 ohms= 375mA with a 3V loss. If it has a 375mA load continuously then it will almost melt with a dissipation of 3V x 375mA= 1.13W. That is its maximum allowed dissipation when the ambient is 30 degrees C.
     
  4. chaos51

    Thread Starter Member

    Jun 18, 2011
    39
    1
    Yes, the circuit uses a LM386, and I am using the one as well in my circuit.. I just got the two numbers mixed up when I wrote the post.

    Ok, I did not notice any overheating there, and I am running 9 volts. I think I better measure the milli-amps going actually through the motors.

    Where do you get the 8 Ohm from? I find the peak-to-peak graph, but that displays volts on both axis? (PDF)
     
  5. Audioguru

    New Member

    Dec 20, 2007
    9,411
    896
    The graph shows the max output peak-to-peak voltage on the left and shows the supply voltage at the bottom.
    It has a bottom line for a 4 ohm speaker where the output is a little less than 4v p-p, a middle line for an 8 ohm speaker, an upper line for a 16 ohm speaker and a top line for no load.

    With a 9V supply and an 8 ohm speaker its output is 6V p-p which is 3V peak. 3V/8 ohms= 375mA but with sounds it is not continuous.
     
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